Mike Pelfrey has been a confusing pitcher to watch this year. At the all star break, he looked terrific and many Met Fans felt he was snubbed for a spot on the All-Star team. Then he hit a rough patch (well, he hit the rough patch a few starts before the all-star break, but you get the point). Then he seemed to get a few good starts here and there. But all in all, he has not panned out as we thought he would after a great April and May. Pelfrey’s xFIP (a measure of the actual skill-level of his performance) for each month has been in order: 4.29, 3.85, 4.15, 5.81, 4.67, and 5.12. You should note that the first 3 of those xFIPs are pretty good, while the last three are mediocre to just plain bad.

Making matters more interesting, Pelfrey’s pitch repertoire has changed this year: He added a split-finger fastball in a move that seemed to provide dramatic success in the early season. He also has maintained his slider and curveball from last season and has two fastballs (a four-seam and two-seam fastball) to go along with this new split-finger fastball. Which of these pitches is responsible for his erratic performance this year? Is his poor performance from July onwards the result of him doing something different? Or simply pitches not being as effective as they were earlier?

– garik16, Amazin’ Avenue

Garik16 does an excellent job here breaking down a ton of pitchFX data that might look overwhelming to sabermetricians lost in the early days of DIPS. I urge you to check it out and read the text, even if the graphics appear a bit scary.

The long and short of it: Pelfrey’s split-finger didn’t fool nearly as many hitters in July, August and September as it did in April, May and June, which probably shouldn’t be too big a surprise. But it’s cool to see it quantified.

Pelfrey’s 2010 peripherals will ultimately look a lot like they did in 2008 and 2009. He’ll finish with around 5 K/9 and 3 BB/9, nothing special, and a groundball rate just shy of 50 percent. His ERA is back down around where it was in 2008, when everyone was heralding the coming of a new ace, and his win total is up to a new career high, but even with the new pitch he has remained consistently unspectacular.

I think a lot of Mets fans still hope for more from Pelfrey because he was such a high draft pick and so highly touted coming through the system, but three seasons of league-average production is nothing to sneeze at from any young pitcher, and really as much as anyone could reasonably hope for from a pitching prospect.

And I think what gets missed a lot when discussing Pelfrey’s value — and a lot of similar pitchers, for that matter — is that he has been healthy and able to make 30+ starts in each of the past three seasons. Even if most of the starts are hardly dominant, just finding guys who can be relied on to provide 200 league-average innings every year is no easy task.

Barring unforeseen circumstances, the world’s best defense or a massive run of good luck, Pelfrey is unlikely to pitch like an ace for any great length of time anytime soon, or, probably, anytime ever. But that he has thus far proven durable and is under team control through arbitration for the next three seasons means he’s a valuable guy to have around as long as we can stomach the incessant speculation about his mental health.